This application relates to a method and apparatus for controlling microwave ovens and particularly to such method and apparatus for controlling the time interval of operation of a microwave oven to achieve a desired temperature within a load material such as food placed within the oven.
Heretofore, several approaches to oven control have been utilized. For example, it is common to utilize a timer which is set by the oven operator based on past experience with a given food. This often leads to error because of the amount of power delivered to a given amount of food varies with its character, quantity, and starting temperature. These are factors which are difficult for the operator to assess. Approaches to solving this problem include approximating the degree of temperature rise in food by various procedures such as incorporating a cooperative or analog element with the food which heats at roughly the same rate and, therefore, can give an indication of the temperature at which the food has achieved. An example of such a system is disclosed in U.S. Pat. No. 3,854,022, issued Dec. 10, 1974, to Donald G. Moore, and entitled "Electromagnetic Oven System for Automatically Heating Variable Numbers and Sizes of Food Items or the Like", in which a ferrite element is incorporated in a food tray, the element being selected to have a Curie transition temperature corresponding to the desired temperature of the food so that upon crossing the Curie temperature the ferrite loses its magnetic properties. This is sensed by suitable magnetic circuit for shutting the oven off. Other systems have included measuring the temperature of the food directly with a pyrometer; measuring the temperature of an associated substance of similar or relatable character; measuring the heat of air or the amount of water vapor coming off the food being heated; and the inclusion of a thermometer probe in the food itself. These systems are generally cooperative in that they sense directly the temperature of the food or measure a property of a material which is an analog of the food. However, they each suffer from certain disadvantages. The utilization of temperature sensors has been found not only expensive, but somewhat unreliable while the direct insertion of a thermal measuring device is impractical, particularly where repetitive use of the oven is required. The use of analog devices permanently installed in the oven involves thermal hysteresis since the analog device may not begin at the same temperature in any given heating cycle. The utilization of a ferrite analog of the food works well in those applications requiring the utilization of a food tray such that the ferrite is at the same starting temperature as the food, as in various institutional environments (i.e., hospitals, inflight meal service). However, for more general applications and, particularly in applications wherein different types and quantities of foods are dispensed from a vending machine for re-thermalization, the expense of providing an analog element at the same temperature as the food during each heating cycle becomes a considerable disadvantage. There is, therefore, a need for a new and improved system for controlling microwave ovens in the heating of foods.
In the present discussion, particular reference will be had to the heating of foods by example; and the models and descriptions given herein will be especially applied to the re-thermalization of previously prepared foods. In certain portions of the present discussion, food equivalent will be represented by given amounts of water. It should be understood, however, that the term -- food (or water)-- as used herein refers to such by way of specific analysis and example, but the term also includes any material which is heatable in a microwave oven. And, the term -heating- should be taken to include not only re-thermalization of such materials by way of example, but to also include the cooking of the same.